Hybrid dunes are a coastal flood defense structure that combines the sandy and wave- dissipating capacity of natural dunes with the robustness of hard structures. Their increasing application in coastal environments highlights the need for a better understanding of how these systems respond to storm forcing. This study investigates the morphodynamic behavior of hybrid dunes under storm conditions through a large-scale field experiment at the Sand Engine (NL), aiming to improve predictive insight into erosion mechanisms and structural performance. High-resolution LiDAR, pressure sensors, and acoustic instruments were deployed across four engineered dune configurations, ranging from a fully sandy dune to hybrid setups with embedded sea dike and seawall cores, to monitor morphological changes and hydrodynamic forcing during five consecutive storms.
The main research question is: What are the dominant erosion mechanisms and structural influences of embedded hard elements in hybrid dunes during consecutive storm events?
The findings show that hybrid dunes transition through three consistent erosion phases: (1) initial sand-dominated retreat characterized by notching, slumping, and offshore sediment transport, (2) reduced erosion once hard elements become partially exposed, and (3) structure-dominated stability after full exposure, during which landward retreat halts. Core geometry strongly influenced behavior: The Dike-in-dune (S1) supported smoother energy dissipation and profile adjustment, while the wall-in-dune (S4) halted retreat more abruptly but caused localized vertical beach erosion. In contrast, the Sandy dune (S2) continued to retreat by over 7 m, with cumulative erosion volumes exceeding 11 m3 /m. Erosion volumes correlated significantly with 20 min time-averaged waterlevel, measured 80 m seaward of the dune foot. Field observation identified the total water level as the dominant driver of erosion. Once exposed, hard structures reduced wave run-up and changed energy distribution, influencing sediment mobility. Oblique wave conditions likely disrupted longshore sediment supply, especially toward unarmored sections, amplifying erosion near structural transitions. These interactions underscore the need to jointly assess cross- and longshore processes when evaluating hybrid dune behavior. Key design factors include sand cover thickness, core geometry, and especially transition zone reinforcement. Structural flanking and undermining were critical failure mechanisms, as seen in the collapse of the Dike-in-Dune and Dike sections during the fourth storm. These results highlight the importance of integrated design strategies addressing not only structural shape, but also lateral sediment continuity and toe stability.
Hybrid dunes shift erosion from retreat-driven to structure-constrained behavior, offering localized resistance under storm conditions. However, their long-term performance depends on robust, systemscale design, balancing sand volume, structural exposure, and sediment pathways to deliver adaptable and resilient coastal protection.

Accumulations of floating debris at culverts and inverted siphons cause partial blockage of the hydraulic structure’s inlet, leading to an increase in head loss that is also referred to as backwater rise. This effect is of high importance in flood management, because backwater rise associated with floating debris accumulations can cause or intensify flooding in the area upstream of the hydraulic structure, yet it is often not adequately addressed in practice or in design guidelines due to a lack of fundamental understanding. A prime example of this was observed at the Geul inverted siphon located near the village of Bunde in the south of Limburg, the Netherlands. During the 2021 floods the inverted siphon experienced a reduction in conveyance capacity due to accumulation of large wood at its inlet, intensifying flooding of nearby villages.

This thesis studied the development of floating debris accumulations at fully submerged and outlet-controlled culverts and inverted siphons, and analysed the effect that floating debris accumulations have on backwater rise. Experiments were performed in a flume under varying conditions of Froude number and submergence rate, achieved by adjusting the upstream flow depth compared to the culvert’s height. The scale model was representative of inverted siphons typical of the Limburg province. Upstream and downstream water levels were measured after addition of fixed batches of debris to the flume. The debris mixture consisted of six size classes of natural wood and was based on logs observed at the Geul inverted siphon. A grate was placed at the culvert’s inlet to prevent logs from passing through. In addition, flow velocities were measured at various locations using Laser Doppler Anemometry.

Based on visual observations and a feasibility study three mechanisms were proposed that describe the development of floating debris accumulations, and the feasibility of these mechanisms was confirmed in preliminary analyses. Furthermore, the experimental results were analysed to determine the influence on backwater rise of canopy drag due to skin friction within and below the accumulation, as well as form drag due to partial blockage of the inlet. It was found that in all cases inlet blockage was the dominant cause of head loss. Using this knowledge a theoretical framework based on a momentum balance equation was derived that relates the inlet blockage ratio to backwater rise, separating the contributions of the grate and debris. A regression analysis was then performed to obtain a design equation for the inlet blockage ratio caused by debris. In conclusion, this thesis provides improved understanding of the development of floating debris accumulations and their influence on backwater rise at submerged culverts and inverted siphons. The results enable more accurate predictions of water levels that can occur during floods and thus offer a solid foundation for flood management strategies.